Synthesis and characterization of a water-soluble carboxylated polyfluorene and its fluorescence quenching by cationic quenchers and proteins.

We have developed a new intermediate monomer, 2,7-[bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(3-(tert-butyl propanoate))]fluorene, that allows the easy synthesis of water-soluble carboxylated polyfluorenes. As an example, poly[9,9'-bis(3''-propanoate)fluoren-2,7-yl] sodium salt was synthesized by the Suzuki coupling reaction, and the properties of the polymer were studied in aqueous solutions of different pH. Fluorescence quenching of the polymer by different cationic quenchers (MV(2+), MV(4+), and NO(2)MV(2+); MV=methyl viologen) was studied, and the quenching constants were found to be dependent on the charge and electron affinity of the quencher molecule and the pH of the medium. The largest quenching constant was observed to be 1.39 x 10(8) M(-1) for NO(2)MV(2+) at pH 7. The change in polymer fluorescence upon interaction with different proteins was also studied. Strong fluorescence quenching of the polymer was observed in the presence of cytochrome c, whereas weak quenching was observed in the presence of myoglobin and bovine serum albumin. Lysozyme quenched the polymer emission at low protein concentrations, and the quenching became saturated at high protein concentrations. Under similar experimental conditions, the polymer showed improved quenching efficiencies toward cationic quenchers and a more selective response to proteins relative to other carboxylated conjugated polymers.